RFC 4616: The PLAIN Simple Authentication and Security Layer (SASL) Mechanism

Network Working Group
Request for Comments: 4616
Updates: 2595
Category: Standards Track

K. Zeilenga, Ed.
OpenLDAP Foundation
August 2006

The PLAIN Simple Authentication and Security Layer (SASL) Mechanism

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Abstract

This document defines a simple clear-text user/password Simple Authentication and Security Layer (SASL) mechanism called the PLAIN mechanism. The PLAIN mechanism is intended to be used, in combination with data confidentiality services provided by a lower layer, in protocols that lack a simple password authentication command.

1. Introduction

Clear-text, multiple-use passwords are simple, interoperate with almost all existing operating system authentication databases, and are useful for a smooth transition to a more secure password-based authentication mechanism. The drawback is that they are unacceptable for use over network connections where data confidentiality is not ensured.

This document defines the PLAIN Simple Authentication and Security Layer ([SASL]) mechanism for use in protocols with no clear-text login command (e.g., [ACAP] or [SMTP-AUTH]). This document updates RFC 2595, replacing Section 6. Changes since RFC 2595 are detailed in Appendix A.

The name associated with this mechanism is "PLAIN".

The PLAIN SASL mechanism does not provide a security layer.

The PLAIN mechanism should not be used without adequate data security protection as this mechanism affords no integrity or confidentiality protections itself. The mechanism is intended to be used with data security protections provided by application-layer protocol, generally through its use of Transport Layer Security ([TLS]) services.

By default, implementations SHOULD advertise and make use of the PLAIN mechanism only when adequate data security services are in place. Specifications for IETF protocols that indicate that this mechanism is an applicable authentication mechanism MUST mandate that implementations support an strong data security service, such as TLS.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [Keywords].

2. PLAIN SASL Mechanism

The mechanism consists of a single message, a string of [UTF-8] encoded [Unicode] characters, from the client to the server. The client presents the authorization identity (identity to act as), followed by a NUL (U+0000) character, followed by the authentication identity (identity whose password will be used), followed by a NUL (U+0000) character, followed by the clear-text password. As with other SASL mechanisms, the client does not provide an authorization identity when it wishes the server to derive an identity from the credentials and use that as the authorization identity.

The formal grammar for the client message using Augmented BNF [ABNF] follows.

   message   = [authzid] UTF8NUL authcid UTF8NUL passwd
   authcid   = 1*SAFE ; MUST accept up to 255 octets
   authzid   = 1*SAFE ; MUST accept up to 255 octets
   passwd    = 1*SAFE ; MUST accept up to 255 octets
   UTF8NUL   = %x00 ; UTF-8 encoded NUL character
   
   SAFE      = UTF1 / UTF2 / UTF3 / UTF4
               ;; any UTF-8 encoded Unicode character except NUL
   
   UTF1      = %x01-7F ;; except NUL
   UTF2      = %xC2-DF UTF0
   UTF3      = %xE0 %xA0-BF UTF0 / %xE1-EC 2(UTF0) /
               %xED %x80-9F UTF0 / %xEE-EF 2(UTF0)
   UTF4      = %xF0 %x90-BF 2(UTF0) / %xF1-F3 3(UTF0) /
               %xF4 %x80-8F 2(UTF0)
   UTF0      = %x80-BF

The authorization identity (authzid), authentication identity (authcid), password (passwd), and NUL character deliminators SHALL be transferred as [UTF-8] encoded strings of [Unicode] characters. As the NUL (U+0000) character is used as a deliminator, the NUL (U+0000) character MUST NOT appear in authzid, authcid, or passwd productions.

The form of the authzid production is specific to the application- level protocol's SASL profile [SASL]. The authcid and passwd productions are form-free. Use of non-visible characters or characters that a user may be unable to enter on some keyboards is discouraged.

Servers MUST be capable of accepting authzid, authcid, and passwd productions up to and including 255 octets. It is noted that the UTF-8 encoding of a Unicode character may be as long as 4 octets.

Upon receipt of the message, the server will verify the presented (in the message) authentication identity (authcid) and password (passwd) with the system authentication database, and it will verify that the authentication credentials permit the client to act as the (presented or derived) authorization identity (authzid). If both steps succeed, the user is authenticated.

The presented authentication identity and password strings, as well as the database authentication identity and password strings, are to be prepared before being used in the verification process. The [SASLPrep] profile of the [StringPrep] algorithm is the RECOMMENDED preparation algorithm. The SASLprep preparation algorithm is recommended to improve the likelihood that comparisons behave in an expected manner. The SASLprep preparation algorithm is not mandatory so as to allow the server to employ other preparation algorithms (including none) when appropriate. For instance, use of a different preparation algorithm may be necessary for the server to interoperate with an external system.

When preparing the presented strings using [SASLPrep], the presented strings are to be treated as "query" strings (Section 7 of [StringPrep]) and hence unassigned code points are allowed to appear in their prepared output. When preparing the database strings using [SASLPrep], the database strings are to be treated as "stored" strings (Section 7 of [StringPrep]) and hence unassigned code points are prohibited from appearing in their prepared output.

Regardless of the preparation algorithm used, if the output of a non-invertible function (e.g., hash) of the expected string is stored, the string MUST be prepared before input to that function.

Regardless of the preparation algorithm used, if preparation fails or results in an empty string, verification SHALL fail.

When no authorization identity is provided, the server derives an authorization identity from the prepared representation of the provided authentication identity string. This ensures that the derivation of different representations of the authentication identity produces the same authorization identity.

The server MAY use the credentials to initialize any new authentication database, such as one suitable for [CRAM-MD5] or [DIGEST-MD5].

3. Pseudo-Code

This section provides pseudo-code illustrating the verification process (using hashed passwords and the SASLprep preparation function) discussed above. This section is not definitive.

   boolean Verify(string authzid, string authcid, string passwd) {
     string pAuthcid = SASLprep(authcid, true); # prepare authcid
     string pPasswd = SASLprep(passwd, true);   # prepare passwd
     if (pAuthcid == NULL || pPasswd == NULL) {
       return false;     # preparation failed
     }
     if (pAuthcid == "" || pPasswd == "") {
       return false;     # empty prepared string
     }
     storedHash = FetchPasswordHash(pAuthcid);
     if (storedHash == NULL || storedHash == "") {
       return false;     # error or unknown authcid
     }
   
     if (!Compare(storedHash, Hash(pPasswd))) {
       return false;     # incorrect password
     }
   
     if (authzid == NULL ) {
       authzid = DeriveAuthzid(pAuthcid);
       if (authzid == NULL || authzid == "") {
           return false; # could not derive authzid
       }
     }
   
     if (!Authorize(pAuthcid, authzid)) {
       return false;     # not authorized
     }
   
     return true;
   }

The second parameter of the SASLprep function, when true, indicates that unassigned code points are allowed in the input. When the SASLprep function is called to prepare the password prior to computing the stored hash, the second parameter would be false.

The second parameter provided to the Authorize function is not prepared by this code. The application-level SASL profile should be consulted to determine what, if any, preparation is necessary.

Note that the DeriveAuthzid and Authorize functions (whether implemented as one function or two, whether designed in a manner in which these functions or whether the mechanism implementation can be reused elsewhere) require knowledge and understanding of mechanism and the application-level protocol specification and/or implementation details to implement.

Note that the Authorize function outcome is clearly dependent on details of the local authorization model and policy. Both functions may be dependent on other factors as well.

4. Examples

This section provides examples of PLAIN authentication exchanges. The examples are intended to help the readers understand the above text. The examples are not definitive.

"C:" and "S:" indicate lines sent by the client and server, respectively. "<NUL>" represents a single NUL (U+0000) character. The Application Configuration Access Protocol ([ACAP]) is used in the examples.

The first example shows how the PLAIN mechanism might be used for user authentication.

S: * ACAP (SASL "CRAM-MD5") (STARTTLS)
C: a001 STARTTLS
S: a001 OK "Begin TLS negotiation now"
<TLS negotiation, further commands are under TLS layer> S: * ACAP (SASL "CRAM-MD5" "PLAIN")
C: a002 AUTHENTICATE "PLAIN"
S: + ""
C: {21}
C: <NUL>tim<NUL>tanstaaftanstaaf
S: a002 OK "Authenticated"

The second example shows how the PLAIN mechanism might be used to attempt to assume the identity of another user. In this example, the server rejects the request. Also, this example makes use of the protocol optional initial response capability to eliminate a round- trip.

S: * ACAP (SASL "CRAM-MD5") (STARTTLS)
C: a001 STARTTLS
S: a001 OK "Begin TLS negotiation now"
<TLS negotiation, further commands are under TLS layer> S: * ACAP (SASL "CRAM-MD5" "PLAIN")
C: a002 AUTHENTICATE "PLAIN" {20+} C: Ursel<NUL>Kurt<NUL>xipj3plmq
S: a002 NO "Not authorized to requested authorization identity"

5. Security Considerations

As the PLAIN mechanism itself provided no integrity or confidentiality protections, it should not be used without adequate external data security protection, such as TLS services provided by many application-layer protocols. By default, implementations SHOULD NOT advertise and SHOULD NOT make use of the PLAIN mechanism unless adequate data security services are in place.

When the PLAIN mechanism is used, the server gains the ability to impersonate the user to all services with the same password regardless of any encryption provided by TLS or other confidentiality protection mechanisms. Whereas many other authentication mechanisms have similar weaknesses, stronger SASL mechanisms address this issue. Clients are encouraged to have an operational mode where all mechanisms that are likely to reveal the user's password to the server are disabled.

General [SASL] security considerations apply to this mechanism.

   Unicode, [UTF-8], and [StringPrep] security considerations also
   apply.

6. IANA Considerations

The SASL Mechanism registry [IANA-SASL] entry for the PLAIN mechanism has been updated by the IANA to reflect that this document now provides its technical specification.

   To: iana@iana.org
   Subject: Updated Registration of SASL mechanism PLAIN
   
   SASL mechanism name: PLAIN
   Security considerations: See RFC 4616.
   Published specification (optional, recommended): RFC 4616
   Person & email address to contact for further information:
        Kurt Zeilenga <kurt@openldap.org>
        IETF SASL WG <ietf-sasl@imc.org>
   Intended usage: COMMON
   Author/Change controller: IESG <iesg@ietf.org>
   Note: Updates existing entry for PLAIN

7. Acknowledgements

This document is a revision of RFC 2595 by Chris Newman. Portions of the grammar defined in Section 2 were borrowed from [UTF-8] by Francois Yergeau.

This document is a product of the IETF Simple Authentication and Security Layer (SASL) Working Group.

8. Normative References

   [ABNF]        Crocker, D., Ed. and P. Overell, "Augmented BNF for
                 Syntax Specifications: ABNF", RFC 4234, October 2005.
   
   [Keywords]    Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [SASL]        Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
                 Authentication and Security Layer (SASL)", RFC 4422,
                 June 2006.
   
   [SASLPrep]    Zeilenga, K., "SASLprep: Stringprep Profile for User
                 Names and Passwords", RFC 4013, February 2005.
   
   [StringPrep]  Hoffman, P. and M. Blanchet, "Preparation of
                 Internationalized Strings ("stringprep")", RFC 3454,
                 December 2002.
   
   [Unicode]     The Unicode Consortium, "The Unicode Standard, Version
                 3.2.0" is defined by "The Unicode Standard, Version
                 3.0" (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-
                 61633-5), as amended by the "Unicode Standard Annex
                 #27: Unicode 3.1"
                 (http://www.unicode.org/reports/tr27/) and by the
                 "Unicode Standard Annex #28: Unicode 3.2"
                 (http://www.unicode.org/reports/tr28/).
   
   [UTF-8]       Yergeau, F., "UTF-8, a transformation format of ISO
                 10646", STD 63, RFC 3629, November 2003.
   
   [TLS]         Dierks, T. and E. Rescorla, "The Transport Layer
                 Security (TLS) Protocol Version 1.1", RFC 4346, April
                 2006.

9. Informative References

   [ACAP]        Newman, C. and J. Myers, "ACAP -- Application
                 Configuration Access Protocol", RFC 2244, November
                 1997.
   
   [CRAM-MD5]    Nerenberg, L., Ed., "The CRAM-MD5 SASL Mechanism", Work
                 in Progress, June 2006.
   
   [DIGEST-MD5]  Melnikov, A., Ed., "Using Digest Authentication as a
                 SASL Mechanism", Work in Progress, June 2006.
   
   [IANA-SASL]   IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL)
                 MECHANISMS",
                 <http://www.iana.org/assignments/sasl-mechanisms>.
   
   [SMTP-AUTH]   Myers, J., "SMTP Service Extension for Authentication",
                 RFC 2554, March 1999.

Appendix A. Changes since RFC 2595

This appendix is non-normative.

This document replaces Section 6 of RFC 2595.

The specification details how the server is to compare client- provided character strings with stored character strings.

The ABNF grammar was updated. In particular, the grammar now allows LINE FEED (U+000A) and CARRIAGE RETURN (U+000D) characters in the authzid, authcid, passwd productions. However, whether these control characters may be used depends on the string preparation rules applicable to the production. For passwd and authcid productions, control characters are prohibited. For authzid, one must consult the application-level SASL profile. This change allows PLAIN to carry all possible authorization identity strings allowed in SASL.

Pseudo-code was added.

The example section was expanded to illustrate more features of the PLAIN mechanism.

Editor's Address

Kurt D. Zeilenga
OpenLDAP Foundation

EMail:

          Kurt@OpenLDAP.org

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